The present invention relates to a system for air dehumidification in air conditioners, comprising an expansion stage, a condenser and comprising a water separator upstream of the expansion stage.
Such systems are known in different embodiments. They serve to dehumidify the air supplied to the cabin of, for example, an aeroplane. The dehumidification of the process air is in particular necessary when the aeroplane is at low flight altitude or when the air conditioner is running in ground operation. In all known systems, cold turbine discharge air is used indirectly or directly for this high pressure or low pressure dehumidification process.
FIG. 1 shows such a dehumidification system in accordance with the prior art. It is a dehumidification process with indirect use of the turbine discharge air. With the dehumidification system shown here, pre-compressed compressed air flows through the primary heat exchanger PHX and is subsequently further compressed in the compressor C. This air now flows through the secondary heat exchanger SHX. Both heat exchangers PHX and SHX are arranged in the stagnation air channel and are cooled with ambient air or with stagnation air in in-flight-operation.
The air discharged from the secondary heat exchanger SHX subsequently flows through the reheater REH and is further cooled in the downstream condenser CON. Some of the humidity contained in the air is hereby condensed. Subsequently, the part of the moisture condensed out is separated in the water separator WE. After passing the water separator WE and the cold reheater REH side, the air is led into the turbine T, expanded therein and cooled thereby. The cooled air is led through the cold side of the condenser CON, whereby the air on the warm side of the condenser CON is cooled and some of the humidity is condensed out. Irrespective of this, further turbine stages can follow after this turbine T. In any case, however, cold turbine discharge air is used for condensation purposes.
Such a dehumidification system brings along the disadvantage that an additional component is required in the form of a condenser, whereby an increased system construction space requirements results due to limited flexibility in the component arrangement. A further disadvantage consists of additional pressure losses due to the condenser CON and a reduced thermodynamic system processor efficiency due to the heat transmission at the turbine discharge. A further disadvantage consists of the fact that, as a rule, a complex condenser design (e.g. bypass) is required to reduce or prevent the icing risk or blockade on the warm and cold side. Provision can, for example, be made for the cold condenser inlet side to be heatable to avoid ice accretion. Overall, an increased effort results for the protection against icing due to the condenser CON.
Furthermore, dehumidification systems are known which provide a direct utilisation of the turbine discharge air, with humidity being separated directly after the turbine. Such a system is shown sectionally in FIG. 2.
The water separator WE is disposed downstream of the turbine T. Here, the temperature of the turbine discharge air must be above 0° C. in order to prevent icing in the water separator WE. The condensation of the humidity takes place in the turbine due to the expansion and to the associated cooling, with further turbine stages also being able to follow the water separator WE. If no further turbine is disposed downstream of the water separator WE, then one speaks of low pressure water separation.
The direct utilisation of the turbine discharge air for dehumidification is associated with the following disadvantages. If only one turbine stage is provided, if it is therefore low pressure separation, the lower pressure level at the turbine discharge results in a higher volume flow for the water separation components, for which purpose correspondingly larger components must be provided. This is associated with an unwanted increase in weight and dimensions. A further disadvantage consists of the fact that low pressure water separators require servicing; high pressure water separators, in contrast, do not.
The turbine discharge temperature is limited to values larger than 0° C. to avoid icing. To produce the demanded cooling capacity, this limited temperature must be compensated by an increased rate of flow. This also results in an unwanted increase in weight and construction height.
Provision can further be made to provide a least two turbine stages (high pressure separation). To avoid the disadvantage of the limited discharge temperature with direct utilisation of the turbine discharge air for the water separation, at least two expansion devices have been required up to now, with the dehumidification taking place between the stages. This second stage, however, makes the system more complex and is associated with additional effort (two refrigerating machines or one refrigeration machine with two turbine stages), additional effort and reduced reliability.